1. Field Of The Invention
The present invention relates to the data transport networks and in particular to a method and apparatus for end-to-end connection between an RPR and an MPLS network.
2. Description Of The Prior Art
As known, the Local Area Networks (LAN) are packet-switched networks which are optimized for the data traffic by using the Ethernet technology. Ethernet is one of the most consolidated technologies to interconnect computers in a LAN network and are based on a bus topology.
The data transport networks, such as for instance the RPR (Resilient Packet Ring) networks are known, fit for the optimal utilization of the available band for packet transport in ring networks. The mechanisms for the operation of the RPR networks are under standardization by the IEEE.
The ring technology can be based—for instance—on physical transport layers SDH, SONET or Ethernet, where the frames of the RPR networks are physically transported. A known RPR network is based upon a two counter-rotating rings configuration, respectively identified as Inner Ringlet and Outer Ringlet. Both the rings are used to transport the data and/or control RPR frames among a series of RPR nodes. RPR is a so-called layer-2 technology with respect to the ISO-OSI or TCP-IP layering.
The format of a RPR frame comprises a part of Header and a part of Payload. The part of payload contains the information of the upper layer to be transported. Among the various fields of the RPR Header, there are the following:                Identifier of the RPR destination node;        Identifier of the RPR source node; and        Protocol Type: identifies the protocol which characterizes the following part of RPR frame/packet, namely the information of upper layer in the payload.        
In each node of a RPR ringlet, the RPR Header is read and the RPR destination node is identified: if the identifier of the destination node corresponds to the identifier of the RPR node which has received the packet, the packet is extracted from the RPR network, otherwise it is forwarded transparently without any processing till reaching the RPR destination node. Another intrinsic characteristic of the RPR networks is that of transporting to the destination also the errored frames, unless the error is an obstacle for the intermediate RPR nodes to identify the RPR destination node. The latest shall be free of deciding if the packet is to be rejected or passed to the upper layer, depending on the type and the “heaviness” of the error.
Typically, on each input/output node are laid several ports connected to customers or upper layers. This means that an RPR node receives frames generated by several ports to be introduced into the RPR network and to be transported to destination where, of course, the frames shall be re-assigned to the respective output ports. Unfortunately, the RPR mechanism does not foresee this type of “select” functionality since it considers only a single input and a single output for each RPR node, and this is one of the problems to be addressed by the present invention.
In the “data world”, in particular for meshed networks, it is also known the MPLS technology: MPLS stands for “Multi-protocol” Label Switching, multi-protocol because its techniques are applicable to any network layer protocol.
As a packet of a connectionless network layer protocol travels from one router to the next, each router makes an independent forwarding decision for that packet. Each router independently chooses a next hop for the packet, based on its analysis of the packet's header and the results of running the routing algorithm.
Packet headers contain information needed to choose the next hop. Choosing the next hop can be thought of as the composition of two functions. The first function partitions the entire set of possible packets into a set of “Forwarding Equivalence Classes (FECs)”. The second maps each FEC to a next hop. Insofar as the forwarding decision is concerned, different packets which get mapped into the same FEC are indistinguishable. All packets which belong to a particular FEC and which travel from a particular node will follow the same path (or if certain kinds of multi-path routing are in use, they will all follow one of a set of paths associated with the FEC).
In conventional IP forwarding, a particular router will typically consider two packets to be in the same FEC if there is some address prefix X in that router's routing tables such that X is the “longest match” for each packet's destination address. As the packet traverses the network, each hop in turn reexamines the packet and assigns it to a FEC.
In MPLS, the assignment of a particular packet to a particular FEC is done just once, as the packet enters the network. The FEC to which the packet is assigned is encoded as a short fixed length value known as a “Label”. When a packet is forwarded to its next hop, the label is sent along with it; that is, the packets are “labeled” before they are forwarded.
At subsequent hops, there is no further analysis of the packet's network layer header. Rather, the label is used as an index into a table which specifies the next hop, and a new label. The old label is replaced with the new label, and the packet is forwarded to its next hop.
In the MPLS forwarding paradigm, once a packet is assigned to a FEC, no further header analysis is done by subsequent routers; all forwarding is driven by the labels. This has a number of advantages over conventional network layer forwarding.
The format of a MPLS frame comprises a part of Header and a part of Payload. The part of payload contains the information of the upper layer to be transported. Among the various fields of the MPLS Header, there are the following:                Label: a label is a short, fixed length, locally significant identifier which is used to identify a FEC. The label which is put on a particular packet represents the Forwarding Equivalence Class to which that packet is assigned. Most commonly, a packet is assigned to a FEC based (completely or partially) on its network layer destination address. However, the label is never an encoding of that address.        Reserved: some bits are reserved for particular further use, out of the scope of the present description, and not described in further details.        TTL (Time-To-Live): each packet carries a “Time To Live” (TTL) value in its header, which is set at the generating node. Whenever a packet passes through a router (node), its TTL gets decremented by 1; if the TTL reaches 0 before the packet has reached its destination, the packet gets discarded. This provides some level of protection against forwarding loops that may exist due to misconfigurations, or due to failure or slow convergence of the routing algorithm.        
Up to now the RPR and the MPLS networks are two separated networks and it is not possible to setup a unidirectional or bidirectional path from a customer box (upper layer) connected to an MPLS edge device and another customer box connected to an RPR edge device.
Therefore a problem is how to best interconnect RPR and MPLS networks.